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Register a new userInfluences of ALD Al$_2$O$_3$ on the surface band-bending of c-plane, Ga-face GaN and the implication to GaN-collector npn heterojunction bipolar transistors
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Due to the lack of effective p-type doping in GaN and the adverse effects of surface band-bending of GaN on electron transport, developing practical GaN heterojunction bipolar transistors has been impossible. The recently demonstrated approach of grafting n-type GaN with p-type semiconductors, like Si and GaAs, by employing ultrathin (UO) Al$_2$O$_3$ at the interface of Si/GaN and GaAs/GaN, has shown the feasibility to overcome the poor p-type doping challenge of GaN by providing epitaxy-like interface quality. However, the surface band-bending of GaN that could be influenced by the UO Al2O3 has been unknown. In this work, the band-bending of c-plane, Ga-face GaN with UO Al2O3 deposition at the surface of GaN was studied using X-ray photoelectron spectroscopy (XPS). The study shows that the UO Al2O3 can help in suppressing the upward band-bending of the c-plane, Ga-face GaN with a monotonic reduction trend of the upward band-bending energy from 0.48 eV down to 0.12 eV as the number of UO Al2O3 deposition cycles is increased from 0 to 20 cycles. The study further shows that the band-bending can be mostly recovered after removing the Al2O3 layer, concurring that the change in the density of fixed charge at the GaN surface caused by UO Al2O3 is the main reason for the surface band-bending modulation. The potential implication of the surface band-bending results of AlGaAs/GaAs/GaN npn heterojunction bipolar transistor (HBT) was preliminarily studied via Silvaco(R) simulations.
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$beta$-Ga$_2$O$_3$ is an ultra-wide bandgap semiconductor and is thus expected to be optically transparent to light of sub-bandgap wavelengths well into the ultraviolet. Contrary to this expectation, it is found here that free electrons in n-doped $beta$-Ga$_2$O$_3$ absorb light from the IR to the UV wavelength range via intra- and inter-conduction band optical transitions. Intra-conduction band absorption occurs via an indirect optical phonon mediated process with a $1/omega^{3}$ dependence in the visible to near-IR wavelength range. This frequency dependence markedly differs from the $1/omega^{2}$ dependence predicted by the Drude model of free-carrier absorption. The inter-conduction band absorption between the lowest conduction band and a higher conduction band occurs via a direct optical process at $lambda sim 349$ nm (3.55 eV). Steady state and ultrafast optical spectroscopy measurements unambiguously identify both these absorption processes and enable quantitative measurements of the inter-conduction band energy, and the frequency dependence of absorption. Whereas the intra-conduction band absorption does not depend on light polarization, inter-conduction band absorption is found to be strongly polarization dependent. The experimental observations, in excellent agreement with recent theoretical predictions for $beta$-Ga$_2$O$_3$, provide important limits of sub-bandgap transparency for optoelectronics in the deep-UV to visible wavelength range, and are also of importance for high electric field transport effects in this emerging semiconductor.
The suitability of Ti as a band gap modifier for $alpha$-Ga$_2$O$_3$ was investigated, taking advantage of the isostructural {alpha}-phases and high band gap difference between Ti$_2$O$_3$ and Ga$_2$O$_3$. Films of Ti:Ga$_2$O$_3$, with a range of Ti concentrations, synthesized by atomic layer deposition on sapphire substrates, were characterized to determine how crystallinity and band gap vary with composition for this alloy. The deposition of crystalline $alpha$-(Ti$_x$Ga$_{1-x}$)$_2$O$_3$ films with up to x~5.3%, was demonstrated. At greater Ti concentration, the films became amorphous. Modification of the band gap over a range of ~ 270 meV was achieved across the crystalline films and a maximum change in band gap from pure $alpha$-Ga$_2$O$_3$ of ~1.1 eV was observed for the films of greatest Ti fraction (61% Ti relative to Ga). The ability to maintain a crystalline phase at low fractions of Ti, accompanied by a significant modification in band gap shows promise for band gap engineering and the enhancement in versatility of application of $alpha$-Ga$_2$O$_3$ in optoelectronic devices.
$beta$-Ga$_2$O$_3$ is a next-generation ultra wide bandgap semiconductor (E$_g$ = 4.8 eV to 4.9 eV) that can be homoepitaxially grown on commercial substrates, enabling next-generation power electronic devices among other important applications. Analyzing the quality of deposited homoepitaxial layers used in such devices is challenging, in part due to the large probing depth in traditional x-ray diffraction (XRD) and also due to the surface-sensitive nature of atomic force microscopy (AFM). Here, a combination of evanescent grazing-incidence skew asymmetric XRD and AFM are investigated as an approach to effectively characterize the quality of homoepitaxial $beta$-Ga$_2$O$_3$ layers grown by molecular beam epitaxy at a variety of Ga/O flux ratios. Accounting for both structure and morphology, optimal films are achieved at a Ga/O ratio of $sim$1.15, a conclusion that would not be possible to achieve by either XRD or AFM methods alone. Finally, fabricated Schottky barrier diodes with thicker homoepitaxial layers are characterized by $J-V$ and $C-V$ measurements, revealing an unintentional doping density of 4.3 $times$ 10$^{16}$ cm$^{-3}$ - 2 $times$ 10$^{17}$ cm$^{-3}$ in the epilayer. These results demonstrate the importance of complementary measurement methods for improving the quality of the $beta$-Ga$_2$O$_3$ homoepitaxial layers used in power electronic and other devices.
The intrinsic performance of type-II InP/GaAsSb double heterojunction bipolar transistors (DHBTs) towards and beyond THz is predicted and analyzed based on a multi-scale technology computer aided design (TCAD) modeling platform calibrated against experimental measurements. Two-dimensional hydrodynamic simulations are combined with 1-D full-band, atomistic quantum transport calculations to shed light on future DHBT generations whose dimensions are decreased step-by-step, starting from the current device configuration. Simulations predict that a peak transit frequency $f_{T,peak}$ of around 1.6 THz could be reached in aggressively scaled type-II DHBTs with a total thickness of 256 nm and an emitter width $W_E$ of 37.5 nm. The corresponding breakdown voltage $BV_{CEO}$ is estimated to be 2.2 V. The investigations are put in perspective with two DHBT performance limiting factors, self-heating and breakdown characteristics.
Gallium oxide films were grown by HVPE on (0001) sapphire substrates with and without $alpha$-Cr$_2$O$_3$ buffer produced by RF magnetron sputtering. Deposition on bare sapphire substrates resulted in a mixture of $alpha$-Ga$_2$O$_3$ and $epsilon$-Ga$_2$O$_3$ phases with a dislocation density of about $2cdot10^{10}$ cm$^{-2}$. The insertion of $alpha$-Ga$_2$O$_3$ buffer layers resulted in phase-pure $alpha$-Ga$_2$O$_3$ films and a fourfold reduction of the dislocation density to $5 cdot 10^9$ cm$^{-2}$.
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